E.M.I. shielding is for providing protection against interference from electromagnetic energy, including radio frequency interference (R.F.I.), which protection may be to protect an electronic device from external E.M.I. and R.F.I. or may be to prevent E.M.I. or R.F.I. from escaping from an electrical device, or both. Environmental sealing is for protection against moisture, corrosion, dust, insects and similar environmental contamination encountered by electrical or electronic devices when in service.
E.M.I. gaskets and seals have been made from the combination of rubber or elastomeric materials and wires or metal filaments. These gaskets are intended to function by the wires contacting the surfaces between which the gasket is assembled to provide the E.M.I. shielding and the rubber or elastomeric material is intended to provide the environmental seal when compressed between the intended surfaces. Examples of such gaskets are disclosed in U.S. Pat. Nos. 2,885,459 to Pulsifer et al.; 3,019,281 and 3,026,367 to Hartwell; 3,126,440 to Goodloe; 3,431,348 to Nellis et al.; 3,542,939 to Mintz; 3,566,064 to Nellis; and 3,708,871 to Mintz.
In another form, E.M.I. gaskets have been provided in the form of a plastic binder containing electrically conductive particles, such as aluminum, coated with silver. Examples of this type of gasket are disclosed in U.S. Pat. Nos. 3,583,930 to Ehrreich et al. and 4,434,541 and 4,507,359 to Powers.
The disclosures of the above references are incorporated herein by reference.
In another form, E.M.I. gaskets have been provided with a flexible foam rubber core having a wire exterior cover on the foam core such as a knitted or braided or otherwise woven wire fabric cover. The foam rubber core is intended to provide resilience to assure that the wire cover makes good electrical contact when compressed between the intended surfaces and is also intended to provide an environmental seal between the intended surfaces.
It has been found that the above gaskets exhibit a number of problems in actual use, including corrosion or rusting of the metallic or wire portion and failure of the plastic, rubber, elastomer or foam to provide the desired environmental seal. In an attempt to eliminate corrosion and rusting, some gaskets have been made of noble metal wires, such as silver, which adds significantly to the cost of the gasket and does not always solve the corrosion and oxidation problems. In some of the above gaskets, a moisture leak path exists where the wires contact the surfaces because the rubber, elastomer or foam material does not adequately seal around and between the wires or metallic structures and the surfaces to prevent migration of moisture into or through the gasket area. Any migration of moisture into or through the gasket area frequently causes corrosion of the metal in the gasket and corrosion or other problems in the electrical or electronic device being protected by the gasket.
It has been recognized in the art that the above gaskets do not fulfill the desired functions. Frequently when the gaskets provide satisfactory E.M.I. shield, they do not provide the desired environmental sealing and/or the gasket itself has corrosion problems. Alternatively when the gaskets provide adequate environmental sealing, they frequently do not provide the desired E.M.I. shielding. Consequently, it has also been recognized in the art that in many cases it has been necessary to separate the two functions and have an exterior conventional environmental gasket, such as an elastomeric or rubber O-ring, and a separate interior E.M.I. gasket, such as a wire mesh. This arrangement is undesirable, however, because it takes extra space, requires extra design, engineering and machining and increases installation difficulties, costs and risks of improper installation.
In view of the above, it is an object of this invention to provide an E.M.I. gasket which provides electrical shielding and provides environmental sealing for not only the space between two surfaces but also for the metallic components which provide the electrical shielding in the gasket. Another object of this invention is to provide a corrosion resistant gasket which need not be made of nobel metal components. Another object of this invention is to provide a method of E.M.I. shielding and environmentally sealing electrical or electronic components using such a gasket.